Most television display systems currently in use employ kinescopes as display devices. The screen or G2 grid, interposed between the control grid and the focusing electrodes, influences a number of significant kinescope operating parameters. Incorrect adjustment may adversely affect the nominal black level of the picture, the color temperature, the kinescope driver amplifier dissipation and the spot size or sharpness capability.
In more detail, in a color kinescope the black level voltage of each electron gun is related to the magnitude of the bias voltage applied to the kinescope G2 (screen) grid. A color kinescope with an "in-line" electron gun structure has a G2 grid electrode energized in common to all three electron guns while a kinescope with separate "delta-type" electron guns has separately energized G2 grid electrodes for each gun. In either case, the G2 bias voltage is often set at a value between 400 and 800 volts, illustratively, so that a desired cathode-to-G1 (control) grid voltage produces a black level condition.
High brightness and high resolution in a reproduced image require a high kinescope peak beam current capability and a small spot size. For each gun, peak beam current capability increases with increasing black level voltage, which is related to the G2 bias voltage. The need for high brightness and high resolution suggests that the highest available G2 bias voltage should be used, consistent with other requirements and constraints on the receiver design.
Some television receivers also employ automatic kinescope bias (AKB) control systems for maintaining a desired black level kinescope cathode bias. Such systems operate to maintain desired cathode-to-G1 bias for each electron gun, and should be capable of operating over a range of black level bias voltages at least as great as the maximum difference in black level voltage between any two kinescope electron guns, which, illustratively, may be on the order of 50 volts or so. To compensate for other system parameter tolerances as well, the operating range of a typical AKB system may be as great as 100 volts. The choice of an operating point within that range is determined by the G2 grid bias voltage. Consequently, it is conventional practice to manually adjust the G2 bias voltage on each receiver to insure that the black level bias voltage of each kinescope electron gun is within the operating range of the AKB system. Furthermore, to obtain high brightness and resolution in a displayed image, the G2 bias voltage should be adjusted so as to make the black level bias voltage of the electron gun with the highest (i.e., most positive) black level voltage nearly equal to the highest usable black level bias voltage capable of being produced by the kinescope driver stages.
When adjusting the G2 screen grid bias, it is also desirable to assure that adequate dynamic range in the blacker than black direction exists for image blanking reserve purposes, particularly in the presence of variations of the operating supply voltage for the kinescope driver circuits.
A known approach to G2 bias adjustment is described by Tallant in U.S. Pat. No. 4,584,596 entitled TELEVISION RECEIVER ALIGNMENT SYSTEM which issued Apr. 22, 1986. The adjustment is manually performed in the receiver service mode and requires modification of the normal bias voltages of both the cathode and the control grid G1 to produce a predetermined cathode-to-G1 voltage. In an illustrative embodiment, the receiver is placed in a service mode which disables the output transistor of the kinescope driver, concurrently disables the automatic kinescope bias (AKB) circuitry and applies an elevated bias voltage (e.g. about double the normal value) to the kinescope control grid G1 whereupon the G2 adjustment is made manually by setting a screen voltage potentiometer.